Electron beam tube having post deflection lens

ABSTRACT

An electron beam tube of the return beam type in which an electron lens is cooperative with the tube deflection system to achieve wide field high resolution operation. The lens is disposed in axial spaced relationship with the deflection system and adjacent a target surface with the focal point of the lens substantially coincident with the center of deflection of the deflection system. The lens provides descanning of the returned electron beam and can provide deceleration of the beam as well as deflection into a path normally incident upon the target for precise electron reflection therefrom.

United States Patent 1 91 Glenn, Jr. et a1.

1451 Aug. 19, 1975 [541 ELECTRON BEAM TUBE HAVING POST $124,790 3/1964 Kuehler 313/80 DEFLECTION LENS 3,307,061 2/1967 Schlesinger.. 313/78 3,796,910 3/1974 Ritz 315/17 [75] Inventors: William E. Glenn, Jr.. Stamford;

tz i New Primary Etaminer-MQ nani R. Wilbur d O O on Aszvislan! E.\arrzir1crJ. M. Potenza [73] Assignee: CBS Inc., New York, NY Attorney, Agent, or FirmSpencer E. Olson, Esq. [22] Filed: Mar. 12, 1973 21 Appl. NO; 340,154 [57] ABSTRACT Related Us A cation Data An electron beam tube of the return beam type in v I pp which an electron lens is cooperative with the tube de- 162] 21 of 158L132 July flection system to achieve wide field high resolution operation. The lens is disposed in axial spaced relationship with the deflection system and adjacent a targ 'i f 315/17 323'2533 get surface with the focal point of the lens substang 'l i 5/ IT tially coincident with the center of deflection of the I l R 30 5 deflection system. The lens provides descanning ofthe 414 returned electron beam and can provide deceleration of the beam as well as deflection into a path normally incident upon the target for precise electron reflection 56] References Cited therefrom.

UNlTED STATES PATENTS 3,040.205 6/1962 Walker 313/65 8 Clams 2 Drawmg Figures IO KV .o 1

SWEEP PATENTED NIH 91975 312m 2 BF 2 FIG.2

ELECTRON BEAM TUBE HAVING POST DEFLECTION LENS This is a division of application Ser. No. l59.l32. filed July 2, l97l now abandoned.

FIELD OF THE INVENTION This invention relates to electron beam tubes and more particularly to an improved tube of the return beam type and capable of operation over a wide field with high resolution.

BACKGROUND OF THE INVENTION In an electron beam tube of the return beam type, an electron beam is scanned across a charge pattern formed on the target surface, the electron beam being returned from the target surface substantially along the incident path to a point near the electron source for detection. In a typical return beam structure, such as employed in an image orthicon camera tube, the primary electron beam from an electron gun is caused to land at the target surface with zero or near-zero energy to allow detection of the target potential, and is also caused to land with normal incidence on the target surface to cause the return beam to fall within the detection area. The charge pattern scanned by the primary beam causes variations in the density of the beam returned from the target surface, these variations being converted to a variable amplitude output signal by a detector receiving the return beam.

Deceleration and normal landing of the primary beam are conventionally accomplished by use of a mesh or slit electrode disposed across the gun structure in a position near the target surface and in the path of the scanned electron beam. While providing satisfactory operation for some purposes, the mesh electrode presents major disadvantages in instances requiring a higher degree of performance. Since the mesh is disposed across the tube in the path of electron flow, resolution is necessarily limited by the size of the mesh openings. In addition. the mesh is subject to vibration which can cause microphonics, and presents an unwanted surface off which electron scattering can occur. Moreover, precision mesh requires highly sophisticated and costly fabrication and is especially difficult to manufacture with large areas. As a result, tube cost is increased by use of such a mesh electrode and the operative field of the tube limited.

SUMMARY OF THE INVENTION In accordance with the present invention, a return beam tube is provided in which wide field high resolution operation is achieved by use of an electron lens cooperative with the tube deflection system and which provides descanning or immobilization of the return beam without any structure in the electron path. Briefly. the invention comprises an electron beam tube having a lens disposed adjacent the target surface and axially adjacent the deflection system with a focal point substantially coincident with a center of deflection of the deflection system. The lens is of the electrostatic type and can be a simple or complex configuration, and can be easily constructed of sufficient aperture to provide a wide scanning field.

The novel tube is typically employed as an image orthicon. SEC vidicon or other camera tube, but is also useful more generally in the scanning of the charge distribution on a target surface, such as in a scanning electron microscope. As an alternative. the invention can also be utilized to impart a predetermined charge distribution on a target surface rather than for determining the charge distribution of such a surface. The invention can provide deceleration of the electron beam for landing at the target surface with zero or near zero energy and normal incidence thereon, as usually required in a camera tube, and can also provide beam landing at a target surface with predetermined higher energy levels. Although the invention is being described herein for use in electron beam tubes, it will be appreciated that the invention is also useful with charged particles other than electrons.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cutaway pictorial view of an electron beam tube embodying the invention; and

FIG. 2 is a schematic elevation view of the embodiment of FIG. I.

DETAILED DESCRIPTION OF THE INVENTION The invention as embodied in a typical return beam tube structure is illustrated in cutaway pictorial view in FIG. 1. The tube includes an electron gun I0 operative to direct a well defined and relatively high density electron beam along the longitudinal axis of the gun, and a return beam detector I2, such as a semiconductor electron multiplier, having an opening 14 centrally provided therein to permit passage of the electron beam from gun I0. A cylindrical focusing electrode 16 is symmetrically disposed about the tube axis adjacent detector 12, and an electrostatic deflection system, including a pair of confronting plates I8 and 20 and an orthogonally arranged pair of confronting plates 22 and 24, is provided symmetrically about this tube axis, as illustrated, for appropriate deflection of the electron beam propagating therethrough. A target 30 having a target surface 28 is provided to receive the electron beam being scanned thereacross by means of the deflection plates.

In accordance with the invention, an electrostatic electron lens 26, which in the illustrated embodiment is in the form of a conductive cylinder symmetrically disposed around the tube axis, is disposed between the target surface 28 and the deflection plates and bears a precise relationship with the deflection system to provide intended operation. More specifically. the lens 26 has a focal point which is substantially coincident with the center of deflection of the deflection plates 18, 20, 22, and 24, and produces an electrostatic field operative to divert the deflected beam into a path substantially normally incident upon target surface 28 and also to decelcrate the beam to a zero or near-zero energy level at the target surface. In contrast to a mesh electrode which is employed in conventional return beam tubes across the path of electron flow, the lens 26 is disposed in surrounding relation to the electron path and thus offers no obstruction to electron flow. That is, the surface of the target is unobstructed by any mesh or other form of electrode which would affect the flow of electrons toward or away from the target surface. For convenience, this absence of obstruction to the electron beam is referred to in the claims by the statement that the target has "an unobstructed surface" for receiving an electron beam scanned thcreacross.

The electron gun can be of many well known configurations to provide the desired beam of electrons. A preferred electron source implementation useful in the present invention is the subject of copending application Ser. No. 63,226, filed Aug. 12, 1970 now U.S. Pat. No. 3,694,687, entitled Electron Gun With Anode Segments for Beam position Detection and assigned to the assignee of the present invention. As described in the above copending application, the electron source includes a cathode, a control grid and an anode structure having a plurality of mutally insulated segments defining an opening for the passage of electrons. each anode segment being connected via a respective resistor to ground potential. The anode segments are shaped to intercept the electron beam if it is off center, and upon such beam interception, the segment intercepting the beam is charged negatively and is effective to deflect the beam toward the center of the source aperture. In this manner the beam is automatically centered and emanates from the source aperture in a well defined position which is in alignment with the longitudinal axis of the tube.

The focusing electrode 16 and deflection plates 18, 20, 22, and 24 are operative with suitable energizing potentials to provide, in a well known manner, focusing and deflection of the electron beam passing therethrough. By virtue of lens 26 cooperatively disposed with respect to the center of deflection of the deflection plates, the path of the beam from the deflection system is modified to provide the desired normal incidence of the beam onto target surface 28 and to decelerate the beam as it approaches the target surface such that the electrons arrive at this surface with zero or at least very low energy. Reduction of the landing energy of the electrons upon target surface 28 is generally desirable in return beam tubes to reduce secondary emission from the target surface which could alter the charge potential on this surface being scanned by the electron beam, and thereby enabling more accurate de tection of the target potential.

According to the invention, the lens 26 and the deflection system are cooperatively arranged to provide a descanned return beam at the detection surface. In the case of the return beam tube described above, where the primary beam lands at the target surface with normal incidence and zero energy, and resulting in an essentially identical primary and return beam path, placement of lens 26 with its focus substantially coincident with the center of deflection of the deflection system achieves intended descanning of the return beam. If, however, the return beam is not at the same energy level as the primary beam, with the return beam path being somewhat different than the forward beam path, the center of deflection is not the same in the forward and reverse directions. Descanning is accomplished in this latter case by placement of the focal point of lens 26 substantially coincident with the center of undeflection of the deflection system, that is, the point in the return path at which the return beam is deflected onto the tube axis for transmission to detector 12.

The tube structure of FIG. 1 is housed within an evacuated enclosure typically in the form ofa glass envelope 32 with electrical connections being made to the tube elements by way of electrical leads (not shown) extending from a mounting socket 34 disposed at the neck 01 the gun adjacent the electron source 10. The elements of the tube are. of course, mounted in accurate spatial relation to each other by suitable mounting hardware within the envelope, as is well known. If the tube is employed as a camera tube. a photocathode will additionally be provided to convey a charge pattern onto target representative of the optical image of a scene being viewed.

Operation of the tube will now be described in con- 10 junction with the schematic elevation view of FIG. 2.

The primary electron beam 36 emitted by gun 10 along the tube axis is focused by the field between electrode 16 and the four deflection electrodes 18, 20, 22 and 24, and is deflected along one axis by an electrostatic deflection field produced by deflection plates 22 and 24 which are suitably energized to provide the intended degree of beam deflection. The orthogonally arranged deflection plates 18 and 20 are not illustrated in FIG. 2 but it will be appreciated that these plates are similarly operative to provide deflection of the electron beam along an axis orthogonal to the deflection axis of the plates 22 and 24.

In the illustrated embodiment, the deflection plates are of symmetrical configuration and essentially have a common center of deflection 38 on the tube axis for both deflection axes. As described, this center of deflection is also substantially at the focal point of lens 26. By operation of lens 26, the deflected beam 40 is diverted into a path 42 normally incident upon target surface 28 and is decelerated to zero or near-zero landing energy at this target surface. The return beam 44 follows a path which is substantially the same as the primary electron beam path and is somewhat broadened to arrive at an area on the receiving surface of detector I2 immediately surrounding the aperture 14. An output signal E is developed across a load resistor R of a magnitude which varies in accordance with the varying density of the return beam caused by the charge pattern on surface 28 scanned by the electron beam.

In the illustrated embodiment, the deflection system has essentially a common center of deflection which, according to the principles of the invention, is substantially coincident with the focal point of the electron lens 26 for providing descanning of the return beam at the detector surface. The invention is also useful in an electron beam tube in which the deflection system has a different center of deflection for each deflection axis. In this latter instance, rather than employing a lens of symmetrical configuration having a single focus such as lens 26 described above, an electron lens of nonuniform shape can be provided having a difierent focus for respective axes of deflection such that the respective foei are substantially coincident with correspond ing centers of deflection for each deflection axis.

In a typical implementation of the invention, the following operating potentials can be employed. The cathode of electron gun I0 is at lOKV, with the anode thereof and focusing electrode 16 at zero or ground potential. A steady state voltage of 8KV is applied to plates 18, 20, 22 and 24 with a deflecting voltage of up to 300 volts across respective pairs of confronting plates to yield a l.5 inch scan at the target surface 28. A potential of 9.4KV is applied to lens 26, while a 9.8KV potential is applied to the target 30. With the above potentials, the return beam is at a 200 volt lower energy than the primary beam, and with lens 26 having its focus substantially coincident with the center of undeflection" of the deflection plates, descanning of the return beam is achieved at the detector surface, although the primary beam does not necessarily land with normal incidence at target surface 28. It should be noted that for primary beam landings at a target surface with relatively high energy, significant secondary emission will occur, such emission being normal to the target surface and comprising the return beam.

Although a simple electron lens 26 is illustrated in the above embodiment, the invention can also be practiced by use of a complex lens construction to suit specific operating requirements. As discussed above, the lens can provide deceleration of the primary beam for zero or near-zero landing energy at the target surface, and can also provide deceleration or acceleration of the primary beam onto the target surface with higher energy levels. It should also be evident that the invention is useful not only for electron beams but more generally with charged particles other than electrons.

Various other modifications and alternative implementations of the invention will occur to those versed in the art without departing from the spirit and true scope of the invention. For example, other deflection and/or focusing systems, such as cylindrical electrostatic deflection electrode arrangements, can be employed in addition to the electrode elements described hereinabove. Accordingly, it is not intended to limit the invention by what has been particularly shown and described except as indicated in the appended claims.

What is claimed is: 1. An electron tube comprising: an elongated envelope having a longitudinal axis; target means supported within said envelope at one end thereof having an unobstructed surface for receiving an electron beam scanned thereacross; an electron gun mounted in said envelope adjacent the other end thereof operative to direct an electron beam along said longitudinal axis; electrostatic focusing means mounted within said envelope adjacent said electron gun and disposed around said longitudinal axis for focusing said electron beam onto the surface of said target means; electrostatic deflection means mounted within said envelope adjacent said electrostatic focusing means and disposed around said longitudinal axis for scanning said electron beam across the surface of said target means, said deflection means having a center of deflection positioned on said longitudinal axis; electrostatic lens means including a conductive structure mounted within said envelope and surrounding said longitudinal axis outside the path of said electron beam for all scanning positions thereof and axially disposed between and in proximity with the surface of said target means and said deflection means; and means for applying to said conductive structure, to

said deflection means and to the unobstructed surface of said target means predetermined potentials having values related to each other to cause the focal point of said electrostatic lens means to be substantially coincident with the center of deflection of said deflection means, to cause an electron beam deflected by said deflection means to be diverted into a path normally incident upon the surface of said target means and to decelerate said normally incident beam to substantially zero velocity at the surface of said target means. 2. An electron beam according to claim 1 further including electron detector means mounted within said envelope intermediate said electron gun and said electrostatic focusing means for receiving a descanned electron beam returned from the surface of said target means.

3. An electron tube according to claim I wherein said electrostatic lens means comprises a conductive hollow cylinder supported coaxially with said longitudinal axis and coactve with said deflection means and the surface of said target means to produce an electrostatic field shaped to divert and decelerate said electron beam.

4. An electron beam tube according to claim I wherein said deflection means includes:

first and second pairs of orthogonally arranged confronting conductive plates and means for applying sweep potentials to said plates for causing electrostatic deflection of said electron beam along a respective deflection axis, said confronting plates having a common center of deflection; and wherein said electrostatic lens means includes a hollow conductive cylinder supported coaxially with said longitudinal axis and which when energized by said predetermined potentials having a focal point substantially coincident with said common center of deflection. 5. An electron beam tube according to claim 1 wherein said deflection means includes first and second pairs of orthogonally arranged confronting conductive plates and means for applying sweep potentials to said plates for causing electrostatic deflection of said electron beam by said first and second pairs of plates along a respective deflection axis, said deflection means having a differ ent center of deflection for each deflection axis;

and wherein the conductive structure of said electrostatic lens means has a non-uniform configuration for causing said electrostatic lens means when energized by said predetermined potentials to have a different focal point for each of said deflection axes, each focal point of said lens means being substantially coincident with a respective center of deflection of said deflection means.

6. An electron tube comprising:

an elongated envelope having a longitudinal axis;

target means supported within said envelope at one end thereof having an unobstructed surface for receiving an electron beam scanned thereacross;

an electron gun mounted within said envelope adja cent the other end thereof operative to direct an electron beam along said longitudinal axis toward the surface of said target means; electrostatic focusing means mounted within said envelope near said electron gun and disposed around said longitudinal axis for focusing said electron beam onto the surface of said target means;

electrostatic deflection means mounted within said envelope adjacent said focusing means and disposed around said longitudinal axis for scanning said electron beam across the surface of said target means, said deflection means having a center of deflection positioned on said longitudinal axis within the volume defined by said electrostatic deflection means;

electron detector means mounted within said envelope adjacent said longitudinal axis and intermediate said electron gun and said focusing means for receiving a descanned return beam from the sur face of said target means;

electrostatic lens means including a cylindrical conductive structure mounted within said envelope coaxially with and around said longitudinal axis outside the path of said electron beam and axially disposed between and in proximity with the unobstructed surface of said target means and said deflection means. the circular area of said cylindrical conductive structure being substantially coextensive with the area of the surface of said target means over which said beam is scanned by said deflection means;

means for applying a potential between said electron gun and said focusing means for accelerating electrons from said gun toward the surface of said target means; and

means for applying to said deflection means, to said cylindrical conductive structure and to the surface of said target means potentials successively more negative with respect to the potential at said focusing means for causing said deflection means to coact with said conductive structure to produce an electrostatic field for diverting an electron beam deflected by said deflection means into a path substantially normally incident upon the surface of said target means and for diverting a return beam from the surface of said target means onto a return path substantially coincident with said longitudinal axis and descanned at said electron detector means.

7. An electron beam tube according to claim 6 wherein said deflection means has a center of undeflection for said return beam and said electrostatic lens means when energized by said potential has a focal point substantially coincident with said center of undeflection whereby said lens means and said deflection means coact to provide descanning of said return beam.

8. A beam tube comprising an elongated envelope having a longitudinal axis;

target means supported within said envelope at one end thereof having an unobstructed surface for receiving a beam of charged particles scanned thereacross;

a charged particle source mounted in said envelope adjacent the other end thereof operative to direct a beam of charged particles along said longitudinal axis toward the surface of said target means;

electrostatic focusing means including a hollow cylindrical conductive structure supported within said envelope adjacent said source and coaxial with said longitudinal axis and means for applying a potential to said conductive structure for focusing said beam onto the surface of said target means;

electrostatic focusing means including a hollow cylindrical conductive structure supported within said envelope adjacent said source and coaxial with said longitudinal axis and means for applying a potential to said conductive structure for focusing said beam onto the surface of said target means;

electrostatic deflection means supported within said envelope adjacent said focusing means and disposed around said longitudinal axis and means for applying sweep potentials to said deflection means for scanning said beam across the surface of said target means, said deflection means having a center of deflection positioned on said longitudinal axis;

electrostatic lens means including a hollow cylindrical conductive member supported within said envelope coaxial with and around said longitudinal axis outside the path of said beam and axially disposed between the unobstructed surface of said target means and said deflection means and means for applying to said conductive member a potential having a value lower than that applied to said focusing means; and

means for applying to said deflection means a potential having a value intermediate the potentials applied to said focusing means and to the conductive member of said electrostatic lens means. said potentials applied to said deflection means and to the conductive member of said lens means being so in terrelated as to produce an electrostatic field operative to cause the focal point of said lens means to be substantially coincident with the center of deflection of said deflection means, to cause a beam deflected by said deflection means to be diverted into a path substantially normally incident upon the surface of said target means and to decelerate said normally incident beam to substantially zero velocity at the surface of said target means. 

1. An electron tube comprising: an elongated envelope having a longitudinal axis; target means supported within said envelope at one end thereof having an unobstructed surface for receiving an electron beam scanned thereacross; an electron gun mounted in said envelope adjacent the other end thereof operative to direct an electron beam along said longitudinal axis; electrostatic focusing means mounted within said envelope adjacent said electron gun and disposed around said longitudinal axis for focusing said electron beam onto the surface of said target means; electrostatic deflection means mounted within said envelope adjacent said electrostatic focusing means and disposed around said longitudinal axis for scanning said electron beam across the surface of said target means, said deflection means having a center of deflection positioned on said longitudinal axis; electrostatic lens means including a conductive structure mounted within said envelope and surrounding said longitudinal axis outside the path of said electron beam for all scanning positions thereof and axially disposed between and in proximity with the surface of said target means and said deflection means; and means for applying to said conductive structure, to said deflection means and to the unobstructed surface of said target means predetermined potentials having values related to each other to cause the focal point of said electrostatic lens means to be substantially coincident with the center of deflection of said deflection means, to cause an electron beam deflected by said deflection means to be diverted into a path normally incident upon the surface of said target means and to decelerate said normally incident beam to substantially zero velocity at the surface of said target means.
 2. An electron beam according to claim 1 further including electron detector means mounted within said envelope intermediate said electron gun and said electrostatic focusing means for receiving a descanned electron beam returned from the surface of said target means.
 3. An electron tube according to claim 1 wherein said electrostatic lens means comprises a conductive hollow cylinder supported coaxially with said longitudinal axis and coactve with said deflection means and the surface of said target means to produce an electrostatic field shaped to divert and decelerate said electron beam.
 4. An electron beam tube according to claim 1 wherein said deflection means includes: first and second pairs of orthogonally arranged confronting conductive plates and means for applying sweep potentials to said plates for causing electrostatic deflection of said electron beam along a respective deflection axis, said confronting plates having a common center of deflection; and wherein said electrostatic lens means includes a hollow conductive cylinder supported coaxially with said longitudinal axis and which when energized by said predetermined potentials having a focal point substantially coincident with said common center of deflection.
 5. An electron beam tube according to claim 1 wherein said deflection means includes first and second pairs of orthogonally arranged confronting conductive plates and means for applying sweep potentials to said plates for causing electrostatic deflection of said electron beam by said first and second pairs of plates along a respective deflection axis, said deflection means havinG a different center of deflection for each deflection axis; and wherein the conductive structure of said electrostatic lens means has a non-uniform configuration for causing said electrostatic lens means when energized by said predetermined potentials to have a different focal point for each of said deflection axes, each focal point of said lens means being substantially coincident with a respective center of deflection of said deflection means.
 6. An electron tube comprising: an elongated envelope having a longitudinal axis; target means supported within said envelope at one end thereof having an unobstructed surface for receiving an electron beam scanned thereacross; an electron gun mounted within said envelope adjacent the other end thereof operative to direct an electron beam along said longitudinal axis toward the surface of said target means; electrostatic focusing means mounted within said envelope near said electron gun and disposed around said longitudinal axis for focusing said electron beam onto the surface of said target means; electrostatic deflection means mounted within said envelope adjacent said focusing means and disposed around said longitudinal axis for scanning said electron beam across the surface of said target means, said deflection means having a center of deflection positioned on said longitudinal axis within the volume defined by said electrostatic deflection means; electron detector means mounted within said envelope adjacent said longitudinal axis and intermediate said electron gun and said focusing means for receiving a descanned return beam from the surface of said target means; electrostatic lens means including a cylindrical conductive structure mounted within said envelope coaxially with and around said longitudinal axis outside the path of said electron beam and axially disposed between and in proximity with the unobstructed surface of said target means and said deflection means, the circular area of said cylindrical conductive structure being substantially coextensive with the area of the surface of said target means over which said beam is scanned by said deflection means; means for applying a potential between said electron gun and said focusing means for accelerating electrons from said gun toward the surface of said target means; and means for applying to said deflection means, to said cylindrical conductive structure and to the surface of said target means potentials successively more negative with respect to the potential at said focusing means for causing said deflection means to coact with said conductive structure to produce an electrostatic field for diverting an electron beam deflected by said deflection means into a path substantially normally incident upon the surface of said target means and for diverting a return beam from the surface of said target means onto a return path substantially coincident with said longitudinal axis and descanned at said electron detector means.
 7. An electron beam tube according to claim 6 wherein said deflection means has a center of undeflection for said return beam and said electrostatic lens means when energized by said potential has a focal point substantially coincident with said center of undeflection whereby said lens means and said deflection means coact to provide descanning of said return beam.
 8. A beam tube comprising an elongated envelope having a longitudinal axis; target means supported within said envelope at one end thereof having an unobstructed surface for receiving a beam of charged particles scanned thereacross; a charged particle source mounted in said envelope adjacent the other end thereof operative to direct a beam of charged particles along said longitudinal axis toward the surface of said target means; electrostatic focusing means including a hollow cylindrical conductive structure supported within said envelope adjacent said source and coaxial with said longitudinal axis and means for applyIng a potential to said conductive structure for focusing said beam onto the surface of said target means; electrostatic focusing means including a hollow cylindrical conductive structure supported within said envelope adjacent said source and coaxial with said longitudinal axis and means for applying a potential to said conductive structure for focusing said beam onto the surface of said target means; electrostatic deflection means supported within said envelope adjacent said focusing means and disposed around said longitudinal axis and means for applying sweep potentials to said deflection means for scanning said beam across the surface of said target means, said deflection means having a center of deflection positioned on said longitudinal axis; electrostatic lens means including a hollow cylindrical conductive member supported within said envelope coaxial with and around said longitudinal axis outside the path of said beam and axially disposed between the unobstructed surface of said target means and said deflection means and means for applying to said conductive member a potential having a value lower than that applied to said focusing means; and means for applying to said deflection means a potential having a value intermediate the potentials applied to said focusing means and to the conductive member of said electrostatic lens means, said potentials applied to said deflection means and to the conductive member of said lens means being so interrelated as to produce an electrostatic field operative to cause the focal point of said lens means to be substantially coincident with the center of deflection of said deflection means, to cause a beam deflected by said deflection means to be diverted into a path substantially normally incident upon the surface of said target means and to decelerate said normally incident beam to substantially zero velocity at the surface of said target means. 